A receiver in a wireless communication system often receives a composite signal that carries multiple, component signals. Some of those component signals may be signals of interest, while others may be interfering signals. As one example, many mobile terminals (“users”) in a cell simultaneously transmit signals of interest to a supporting base station. That base station receives all of these signals of interest, along with any number of interfering signals (e.g., from other cells), together as a composite signal.
Since the receiver receives all of the signals together as a composite signal, the receiver must attempt to separate the component signals from one another in order to recover the signals of interest. Some circumstances significantly impair the receiver's ability to fully separate the component signals. Severe channel conditions, for example, can destroy any orthogonality that may have otherwise existed among the signals of interest. Also, orthogonality between the signals of interest and the interfering signals may not even be possible.
The receiver of a base station in Long Term Evolution (LTE) systems often experiences circumstances like these when attempting to separate control signals received from mobile terminals on the Physical Uplink Control Channel (PUCCH). More particularly, mobile terminals sometimes transmit Hybrid-Automatic Repeat Request (HARQ) acknowledgments (ACK or NACK) and scheduling requests to their supporting base station over the PUCCH. Different terminals can share the same physical resources allocated to the PUCCH, but can be separated through spreading of their respective control signals over time and frequency.
In the frequency-domain, each terminal's control signal is spread with a cell-specific base sequence that has been phase rotated in any number of ways. Terminals using different phase rotations of the same cell-specific base sequence can thus be separated through spreading over frequency. Terminals using the same phase rotation can be separated through spreading over time instead. In this regard, each of the terminals' control signals is spread with a different time-domain cover sequence.
Severe channel conditions destroy orthogonality attained from different phase rotations, and likewise destroy orthogonality attained from different cover sequences. This loss of orthogonality causes intra-cell interference between the terminals' control signals and thus seriously impairs the base station's ability to separate those signals from one another.
Moreover, the cell-specific base sequences used by terminals in one cell are not orthogonal to the cell-specific base sequences used by terminals in a neighboring cell, even under ideal channel conditions. This causes inter-cell interference between terminals transmitting control signals in different cells and thus impairs a base station's ability to separate those signals from one another.
Known approaches to mitigate interference on the PUCCH include randomizing the interference through time-hopping. The particular phase rotation used by a terminal varies on a symbol-by-symbol basis. Also, the particular time-domain cover sequence and/or base sequence used by a terminal varies on a time-slot basis. While time-hopping significantly reduces interference on the PUCCH, some interference still remains on the PUCCH.